1. Field of the Invention
The present invention relates to circuit protection from ESD (Electro Static Discharge, and more particularly to use of SCRs (Silicon Controlled Rectifiers) to protect against ESD events and latch up.
2. Background Information
Conventional ESD protection devices and methods include SCRs coupled across I/O (input/output) nodes that are directly coupled to the external environment and, thus, susceptible to external influences. Such SCRs are also connected across nodes that connect to power supplies. The protecting SCR, when positioned across two nodes (one node typically being ground), will present an open circuit to node voltages (meaning the voltage difference between the two nodes) less than the trigger or breakdown voltage of the SCR. When triggered by a voltage above the trigger voltage the SCR presents a low impedance across the pins as long as the node voltage is above the holding voltage of the SCR. Past emphasis has been placed on making the SCR trigger quickly so that any high voltage ESD event is shunted quickly not reaching a voltage that may damage the electronics.
Herein the words “connect” and “couple” are interchangeable and defined herein as “functionally connected,” meaning other components may or may not be used in the connection.
Typically systems using SCR protection include electronic circuit(s) mounted to a PCB (printed circuit board) with nodes (pins) that connect to power supplies and to (I/O) external systems. Pins or nodes are both defined herein to mean all the various types of connections of a system that connect to the external environment. They are interchangeable herein.
Conventional SCR's are placed across nodes to protect against ESD events. These SCRs are selected with a protection window that defines a high voltage, greater than the operating voltage of the node but below the maximum voltage for the node, and a low holding voltage. The SCR is designed to trigger at the high window voltage and turns off when the voltage lowers below the low holding voltage. For example, an ESD event often is an ungrounded human coming in contact with a node where several thousands volts of static electricity may contact the node. When a typical ESD event occurs the protecting SCR triggers maintaining a low voltage on the node and dissipating the ESD. When the node voltage falls below the low holding voltage the SCR turns off again presenting a high impedance across the node.
Damage to devices typically can be described as a catastrophic voltage breakdown of an oxide layer that destroys the device or as latch up where a device unexpectedly remains turned on. Latch up draws current from the external system and may not destroy the device if the current is limited. As the NMOS devices get smaller, latch up becomes more of an issue. Latch up typically involves parasitic bipolar transistors. These transistors are biased off, but when stressed by over voltage, heat, radiation, etc. the PN junctions may become forward biased in a regenerative manner and draw excessive current. The circuit does not function as designed, but as mentioned above the device may not be damaged.
Generally nodes that that connect to power supplies, e.g., Vcc, are more susceptible to latch up while I/O pins are more susceptible to ESD voltage breakdowns. It would be advantageous to have an SCR protective device across power supply node with a triggering voltage substantially higher (a large gap) that the power supply voltage to protect against latch up. At the same time it would be advantageous to provide an SCR with a triggering voltage only a little above (small gap) the operating voltage of an I/O pins to protect against ESD breakdown and to minimize I/O node leakage. The small gap facilitates these conditions.
Moreover, when a circuit is unpowered, it would be advantageous to still protect the circuit by triggering the protective device (SCR) at a low, e.g. one volt, triggering threshold.